Answers

Louise is right - addition of humus rich materials such as composted manure, soil conditioning compost from the garden centre or local recycling centre, or your own garden compost, spent mushroom compost, leaf mould, anything you can get hold of like that, and add as much as possible, repeating with some every year at least. Horticultural grit in big bags is also very useful.

Adding calcium to the soil, in the form of lime, gypsum, or bone meal, will cause the clay particles to stick together in sand-sized clumps--a process called "flocculation". That works even better in the presence of organic matter. Magnesium, in the form of Epsom salts or dolomite, has a similar effect, but an overdose is more damaging to plants.